Side-Feed Forced-Air Biomass Burning Cookstove

ABSTRACT

Disclosed herein are systems and devices that aid in reducing particulate emissions from biomass stoves, for example by the use of gases injected in or near the oxidation zone of a combustion chamber. Also disclosed are systems and devices providing electricity to a pump or blower that aids in injecting the gas and/or collecting exhaust gases. In some embodiments the device providing electricity is a thermoelectric generator that may also be used to power other devices. In many embodiments, the systems and devices inject a gas at or near the oxidation zone in the combustion chamber of a biomass stove. The gas injected into the zone may be fresh air, exhaust from combustion or combinations thereof. The gas can be forced into the to combustion chamber with the aid of a pump or blower that may also aid in drawing in exhaust gas.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit of priority pursuant to 35 U.S.C. §119(e) of U.S. provisional patent application No. 62/048,884, filed on Sep. 11, 2014, which is incorporated herein by reference in its entirety.

FIELD

The present disclosure systems and devices that aid in reducing emissions from biomass stoves, for example by the use of gases injected into the combustion chamber at or near the oxidation zone. Also disclosed are systems and devices for generating electricity from a biomass stove to operate a pump or blower that aids in injecting the gas and/or collecting exhaust gases.

BACKGROUND OF THE INVENTION

The global commercial market for improved cookstoves is a nascent one. Though there have been decades of improvements in stove programs, the impact of these programs has been limited due to poor durability and performance, or a lack of understanding of the market such as price point, misplaced subsidies, lack of promotion and education. As discussed below are many anticipated public benefits of improved cookstoves.

Health Impact for Rural Masses.

The ambient concentration of airborne pollutants such as CO and particulate matter less than about 10 or 2.5 micrometers (PM₁₀, PM_(2.5)) in the homes in rural areas of developing countries has been observed to exceed World Health Organization (WHO) exposure limits up to 30×, and US Environmental Protection Agency limits by 100×. This indoor air pollution (IAP) has been linked to nearly 3% of the global burden of disease and is a major contributor to as many as 2 million premature deaths each year. Advanced gasifier cookstoves have demonstrated their potential for reduced PM and CO emissions relative to three-stone fires.

Exhaust gas recirculation (EGR) technology may further reduce emissions, substantially benefitting hundreds of thousands of users.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows various embodiments of the presently disclosed EGR cookstoves and devices.

FIG. 2 shows emissions improvements over a 3-stone fire for one embodiment of the disclosed EGR prototype

FIG. 3 shows discreet PM emission data for one embodiment of an EGR stove.

FIG. 4 shows soot volume fraction, primary particle size, and number concentration of primary particle distribution in 30% oxygen volume fraction (a) N₂+O₂, (b) CO₂+O₂, (c) Ar+CO₂+O₂ in co-flow. These are cross sectional “photos” (using Time Resolved Laser Induced Incandescence (TIRE-LII) and TEM photography) of a co-flow non-premixed flame. The fuel is flowing coaxially with three different gas mixtures, whose compositions are noted in the figure subscript.

FIG. 5 shows various embodiments for a power source for a fan-driven EGR system disclosed herein.

FIG. 6 shows pictures of an embodiment of an EGR-enabled biomass stove used for testing.

FIG. 7 shows a schematic of an embodiment of an EGR-enabled biomass stove (top) and a cut-away of the EGR device.

FIG. 8 an embodiment of an EGR-enabled stove for testing emission reduction parameters.

FIG. 9 the embodiment of FIG. 8 shown from the front, where two injection nozzles are placed in the mouth of the stove.

FIG. 10 depicts an EGR device for adding to a biomass stove.

FIG. 11 depicts one embodiment of the disclosed device having a preferred location for an injection nozzle positioned in an EGR enabled stove.

FIG. 12 depicts one embodiment of the disclosed device having a preferred geometry for the injection nozzle positioned in an EGR enabled stove.

FIG. 13 is a picture of one embodiment of the disclosed system and device used for testing the effect that gas temperature has on emissions.

FIG. 14 shows results of a start-up phase flow rate analysis on one embodiment of the disclosed stove.

FIG. 15 shows results of a steady state firepower phase flow rate analysis on one embodiment of the disclosed stove.

FIG. 16 shows various positions of a nozzle for use with the disclosed devices and systems.

FIG. 17 shows analysis of studies testing emissions as a function of temperature.

FIG. 18 shows star-up and steady state emissions as a function of flow rates for various nozzle positions.

FIG. 19 shows results from air injection flow rate optimization for side injection nozzles.

FIG. 20 shows results for tests of optimized PM_(2.5) emissions as a function of nozzle diameter for side injection nozzles.

FIG. 21 shows results for steady state flow velocities and optimized emissions for various diameters injection orifices.

FIG. 22 shows local peak emissions for 3.2 mm nozzles.

FIG. 23 shows local peak emissions for 5.7 mm nozzles.

FIG. 24 shows flow profiles in combustion chamber at varied air injection flows rates and effect on emissions.

FIG. 25 shows various injection locations tested in G3300.

FIG. 26 shows several photos of an embodiment of a chimney ring nozzle.

FIG. 27 shows air-flow rate effect at top injection location.

FIG. 28 shows injection angles tested at bottom of chimney.

FIG. 29 shows an embodiment of an angled chimney ring nozzle.

FIG. 30 shows start-up and steady state PM and flow rate for G3300 with side injection nozzles with 1.5 mm diameter injection orifices.

FIG. 31 shows start-up and steady state PM and flow rate for G3300 with side injection nozzles with 2.3 mm diameter injection orifices.

FIG. 32 shows start-up and steady state PM and flow rate for G3300 with chimney ring at bottom of upper combustion chamber with 1.5 mm diameter injection orifices.

FIG. 33 shows start-up and steady state PM and flow rate for G3300 with chimney ring at middle of upper combustion chamber with 1.5 mm diameter injection orifices.

FIG. 34 shows start-up and steady state PM and flow rate for G3300 with chimney ring high in upper combustion chamber with 1.5 mm diameter injection orifices.

FIG. 35 shows start-up and steady state PM and flow rate for G3300 with chimney ring at bottom of upper combustion chamber with 1.5 mm diameter injection orifices.

FIG. 36 shows start-up and steady state PM and flow rate for G3300 with chimney ring at bottom of upper combustion chamber with 3.0 mm diameter injection orifices.

BRIEF DESCRIPTION

Disclosed herein are systems and devices for reducing emissions from biomass combustion devices (e.g. stoves). The disclosed systems and devices may include an exhaust gas recirculation system (EGR) and/or fresh air injection system that reduces particulate emissions. The disclosed systems and devices may also be used to increase thermal efficiency of the biomass combustion devices (e.g., stoves). In many embodiments where exhaust gas is used for injection, a fraction of the emissions of a biomass-burning system (combustion exhaust) are captured and re-injected into the combustion zone. In some embodiments, exhaust gases maybe combined with fresh air prior to re-injection. Use of the disclosed systems and devices may aid in reducing emissions from the biomass combustion device (e.g. CO and particulate matter) and, in some embodiments, can provide electricity for powering an injection fan/blower and other electronic devices (for example a phone or battery).

Disclosed herein are devices for reducing emissions from a biomass stove, the device comprising: a fluid inlet orifice; an inlet conduit having an outside surface and an interior surface, the interior surface defining an inlet chamber, the inlet chamber in fluid communication with the exterior surface via the inlet orifice, the interior chamber for channeling a fluid (for example a gas, such as air, which may comprise greater than about 15% oxygen, O₂); a fan positioned within the inlet chamber and distal the inlet orifice, the fan for drawing a fluid through the inlet orifice and into the chamber, and into; an outlet conduit the outlet conduit having an interior surface defining an outlet chamber, the outlet chamber in fluid communication with the inlet chamber; one or more nozzles having an interior in fluid communication with the outlet chamber, the nozzle for directing the fluid into a combustion chamber of a biomass stove; and a plurality of outlet orifices defined on the surface of the nozzle, the outlet orifices designed to allow the fluid to exit the interior of the nozzle. In some embodiments, the nozzle is positioned at or near the top of a lower combustion chamber. In some embodiments, the outlet orifices have an average diameter of between 0.5 and 3.5 mm, and define a circle, a square, a triangle, or an oval, the average diameter being measured through the center of the circle, square, triangle, or oval. In some embodiments, the volume of gas escaping the one or more nozzles is greater than about 10 standard liters per minute and less than about 100 standard liters per minute, and may escape the orifice at from about 5-25 meters/second. In some embodiments, the nozzle is linear or circular, such as a circular ring positioned above the lower combustion chamber and within the lower half of the upper combustion chamber, and designed to allow combustion gasses to pass directly through an injection region.

Also disclosed are methods for reducing emissions (for example particulate emissions, in some cases particulates less than about 2.5 micrometers), from about 20% to about 90%, from a biomass stove, the method comprising: placing a gas into an interior chamber of nozzle, the nozzle positioned at or near a flame; increasing the pressure of the gas within the nozzle (for example by using a fan or pump to force the gas into the nozzle interior); expelling a volume of the gas from the nozzle through a plurality of outlet orifices defined by the outer surface of the nozzle; and directing the injected gas into a flame within a combustion chamber of the biomass stove, wherein the gas decreases the amount of at least one pollutant exiting the biomass stove. In some embodiments, the volume of gas expelled from the nozzle(s) is between about 10 standard liters per minute and 100 standard liters per minute. In some embodiments, the nozzle defines a linear tube or circular ring, and the outlet orifices are positioned in the interior surface of the ring to aid in injecting a gas into the center of the ring, said the outlet orifices having a diameter of between 0.5 and 6.0 mm. In some embodiments, the outlet orifices are positioned equidistant from a floor of the combustion chamber, and the gas is expelled through one or more orifices at a velocity of between 5 and 25 meters per second. In many embodiments, the gas is injected into the flame at an angle of between about −10 degrees to about +30 degrees.

Also disclosed herein are methods of reducing particulate emissions from a biomass stove, the method comprising: drawing a gas into a chamber, the gas comprising greater than about 15% O₂; channeling the gas from the chamber into a nozzle having an interior surface and an exterior surface, the nozzle defining a circular tube having a plurality of outlet orifices on the inner surface of the circle, wherein the outlet orifices allow a gas to travel from the interior of the tube and toward the center of the circle; increasing the pressure of the gas within the interior of the nozzle; expelling a volume of the pressurized gas from the nozzle at a velocity of between about 5 meters per second and 20 meters per second; and directing the injected gas into a flame within a combustion chamber of the biomass stove, wherein the gas decreases the amount of at least one pollutant exiting the biomass stove by greater than about 25% relative the stove lacking a nozzle or lacking a pressurized gas within the nozzle.

DETAILED DESCRIPTION

Described herein are stoves and stove accessories that recirculate combustion products back into the combustion chamber. In some embodiments the stoves and stove accessories may mix the combustion products with fresh air prior to introducing them back into the combustion chamber. In some embodiments, the recirculation of exhaust gas into the combustion chamber may provide strong interaction with the otherwise non-premixed diffusion based combustion occurring between the biofuel and the naturally aspirated intake air.

While multiple embodiments of the disclosed recirculation devices and systems are disclosed, still other embodiments will become apparent to those skilled in the art from the following detailed description. As will be apparent, the disclosed systems and devices are capable of modifications in various obvious aspects, all without departing from the spirit and scope of the present invention. Accordingly, the detailed description is to be regarded as illustrative in nature and not restrictive.

The negative impacts of incomplete biomass combustion is damaging to both the local and global environment. Additionally, although biomass fuels are often celebrated for potentially being able to provide carbon-neutral energy, they are not climate-neutral, as a significant fraction of the carbon contained in the fuel is re-emitted as species with high global warming potentials such as CH₄ and NMHC. The cookstove design described herein has the potential to reduce this pollution substantially.

Furthermore, recent research suggests that Black Carbon (BC) particle emissions are the second most important driver of global climate change behind CO₂. Globally, household cooking is estimated to produce 50% of the total anthropogenic BC emitted into the environment each year, and aggressively reducing global BC emissions with technologies such as the cookstove described herein has been promoted as one of the most promising strategies for combating near term global warming.

Global Fuel Savings

It is estimated that biomass stoves account for approximately one eighth of net deforestation and thus about 1.5% of net human CO₂ additions to the atmosphere. On a more regional basis, women and children will spend as much as 20 hours per week gathering firewood, an activity which prohibits their development and in many cases exposes them to violence (Global Alliance for Clean Cookstoves).

There are basically three primary approaches to ultra-clean biomass cookstoves. These approaches are (1) forced draft semi-gasifier stoves, (2) natural draft semi-gasifier stoves, and (3) side-feed fan stoves. Disclosed herein is extensive performance data on various commercially available stoves, and a novel and surprisingly efficient side-feed fan stoves. Based on Envirofit's extensive experience in the marketplace, it has been found:

Forced draft gasifier stoves that require fuel processing do not meet the market expectations around price and convenience.

Operational sensitivities of the natural draft and forced draft semi-gasifier stoves result in the potential for high emissions when (1) fuel quality changes, (2) during start-up and shut-down, and (3) users operate the stove outside of the optimal window.

In contrast, a side feed stove meets consumer's expectations around fuel flexibility and ease of use. In addition, a the stove is allowed to function properly even without the operation of the fan. While conventional rocket stoves do not meet the 90% emissions reduction target, they do meet a reduction up to 70%, thus ensuring reasonable emission reductions even in a failure mode.

As a side-feed fan stove has the greatest potential to meet all aspects of consumer's needs, expectations, and performance, there is accordingly a need for a more efficient side feed fan stove that meets a 90% or greater emissions reduction target. This target helps in achieving the benefits previously described of improving health for the rural masses, reducing global environmental impact, and reducing global biomass fuel use.

Although many of the embodiments described herein utilize commercially available stove, such as the EnviroFit G3300 stove, as a base biomass combustion device, the subject invention can be employed on side-feed stoves and side-feed fan stoves of various design. FIG. 1 shows several commercially available side-feed fan stove designs: Stove Tec (top); BioLite (middle); Envirof it (bottom, with and without adapter)

Exhaust gas recirculation (EGR) in various forms has been used in other situations to control flame properties. EGR is predominantly utilized in internal combustion engines. The primary purpose of EGR in internal combustion engines is the reduction of NOx formation. It does so by introducing somewhat inert exhaust gas into the cylinder, therefore lowering the proportion of combustible gas and distributing the thermal energy over a larger mass. This reduces the peak flame temperature, therefore reducing the thermal decomposition of N₂ and the consequent formation of NOx. But since these engines operate near stoichiometric levels, adding the exhaust gas can make localized regions where there is less than a stoichiometric level of oxygen. This promotes incomplete combustion and therefore generally increases the total production of particulate matter (partially combusted hydrocarbons). Although, it has been found that increasing the concentration of CO₂ can help to mitigate the increased PM production due to EGR.

Considering the effects of EGR on internal combustion engines, it is counter-intuitive to apply EGR to biomass cookstoves for three main reasons. One, NOx formation in biomass cookstoves is not of concern. This is because peak combustion temperatures in the stove are low enough that a negligible fraction of N₂ molecules will thermally decompose. Two, since biomass cookstoves generally have relatively low peak combustion temperatures when compared to engines, the skilled artisan would assume that lowering the temperatures in cookstoves, by introduction of exhaust gases, would promote incomplete combustion and enhance formation of particulate matter. Three, because biomass cookstoves (especially the rocket elbow stove) operate with such high excess O₂ values (excess air to stoichiometric air ratio of about 2.3 for the M5000), the concentration of CO₂ in the cookstove EGR would likely be insignificant compared to the minimum concentration of CO₂ in engine EGR that is needed to help mitigate the issue of PM production.

In contrast to the expected results described above, Applicants have unexpectedly discovered that applying EGR to a biomass cookstove (1) decreases the particulate matter production and (2) increases CO oxidation, as seen in FIGS. 2 and 3. In comparison to EGR combined with combustion engines, the presently disclosed findings are counter-intuitive. Current literature can provide some hindsight based theories for this observed emissions reduction.

As previously mentioned, recirculation of CO₂ into the combustion chamber of biomass cookstoves is one possible mechanism that causes a reduction particulate production, albeit a potentially small one considering the excess air ratios in the rocket elbow stove. The effect of CO₂ addition on particulate matter emissions in non-premixed flames has been documented in the literature. These studies focused on the soot formation in homogenous fuels such as propane and ethylene. Early literature reports that the addition of carbon dioxide caused a reduction in soot formation for a counter flow diffusion flame through chemical interaction. A more recent study indicates that the reduction of particulate matter from the addition of carbon dioxide is due to both a reduction in the flame temperature and chemical interaction of the CO₂. FIG. 4 shows experimental data collected by Oh et al. which clearly demonstrates the reduction in soot obtained with carbon dioxide addition to the oxidizer in a co-flow diffusion flame cross-section. In the figure, fv represents the volume fraction of soot in the flame, dp is the primary particle size, and Np is the number concentration of primary particles in the flame. According to Oh et al, “In the case of using CO₂ as the diluents instead of N₂, the primary particle size and soot volume fraction decrease abruptly.” Again, according to Oh et al, the reduction in soot formation is attributed to the following:

A reduction in flame temperature due to the increased heat capacity of CO₂

Dilution of reactive species from the introduction of carbon dioxide

A direct chemical effect of carbon dioxide

These studies confirm the effect of CO₂ addition on particulate emissions for co-flow non-premixed flames, but no literature currently exists that reaffirms this effect for combustion of solid-biomass fuels. More surprisingly, Applicants have discovered that other gas constituents and the injection characteristics (speed, direction, location, angle, volume) affect PM reduction.

There are other potential mechanisms that can further explain the reduction in particulate emissions, such as increased mixing, increased levels of O₂, change of the total flow through the stove and subsequent change of the residence time of combustible constituents, change of the peak combustion temperature in the stove, and destruction of the recirculated particulate. These mechanisms are neither completely understood nor easily predicted. Currently no literature exists that evaluates the effect of these mechanisms on particulate emissions in biomass cookstoves, especially relating to the application of EGR in biomass cookstoves.

In addition to the potential benefits that EGR has on particulate matter formation, experimental and literature data indicate that reduction of carbon monoxide is possible with EGR through mechanisms such as increased mixing and water in the exhaust stream catalyzing CO oxidation.

In some embodiments, the stove and stove accessory may include a device for actively moving the air, and the device may be powered by a power source. In some embodiments, the air movement device may be a fan or blower. The power source may be a battery, which may include an adapter and/or a charging circuit. Several possible embodiments are shown in FIG. 5. In one embodiment, the power source includes an AC/DC adapter with battery and charging circuit. This embodiment may be desirable for certain markets, for example India, where over 70% of the target market has access to electricity for at least some part of the day. Another embodiment may include a hand-operated generator (or dyno). The hand operated generator may be combined with the charging circuit and battery described above. Hand charging would not be overly burdensome for the anticipated fan power consumption of approximately 1-3 Watts. Also shown in FIG. 5 is a thermoelectric generator (TEG) powered system, which may use heat from the stove to generate in electricity. In many cases, the TEG may generate in excess of 1-3 Watts, which may also allow charging of batteries and/or other electronic devices (lamps, lights, cell phones, computers, etc). The potential to generate excess electricity is attractive. Other options will be apparent to those skilled in the art and are wholly consistent with the subject invention as described herein.

Several embodiments of the disclosed stove and stove accessories are depicted in FIGS. 6-9. The embodiment shown in FIG. 6 includes a commercially available stove, the Envirofit G3300. This embodiment was used in testing many aspects of the claimed stoves and stove accessories. For example, the emissions data shown in FIGS. 2 and 3 was generated with this embodiment.

A second embodiment of the disclosed stove and stove accessory is shown in FIG. 7. This embodiment of an EGR-enabled stove was used to analyze several variables, for example variables that may affect a stove's emission performance, for example gas injection location, nozzle geometry, gas path temperature, flow rates, and TEG location. The embodiment of FIG. 7 also includes a commercially available stove, the Envirofit M5000. Not shown in FIG. 7 are the conduits that direct gas from the EGR outlet to the side feed opening in the stove. The conduits not shown in FIG. 7 are shown in the embodiments depicted in both FIGS. 8 and 9. These conduits direct gas from the EGR outlet to a side feed opening in the stove. A pulse width modulator and power supply can be included in the disclosed device to aid in controlling the speed of the fan/blower motor. In addition, the TEG incorporated into the EGR path of the embodiment of FIG. 7 aided in characterizing the power generation/recovery from the recirculating exhaust gas.

Another embodiment of an EGR-enabled stove is depicted in FIG. 10. This embodiment illustrates how an EGR stove accessory may be added on to a stove as an accessory, for example the EGR device may be added to a rocket elbow stove. In this embodiment, the exhaust gas is drawn in through a grid of inlet holes, which may be positioned at or near the top of the stove, as depicted here. The exhaust gas flows through a conduit that may be positioned about a perimeter of a pot skirt. The exhaust gas within this conduit then flows via one or more addition conduits at or near the front of the pot skirt and is injected back into the mouth of the combustion chamber. A fan, or injection air blower, may be positioned between the inlet holes and the injection holes. In some embodiments, as depicted in FIG. 10, the fan is located at the backside of the pot skirt. A power supply may be positioned near the fan.

As discussed above, the injection air blower may be supplied by electricity provided by a power source such as a thermoelectric generator, solar power cell, hand-powered generator (crank charger), or residential power. The choice of power source may be based on cost evaluations and comparisons against market demand. The materials selected for the stove and stove accessory can vary depending on the thermal, chemical, and mechanical environments to which the materials are exposed. In many embodiments, the components within the device may vary and do not need to be the same or similar to those utilized in the EnviroFit G3300 or M5000 stoves discussed above. In many embodiments, the EGR device may allow the stove to function properly and efficiently when the EGR system is off and/or disabled.

For most embodiments of the disclosed EGR system, the stove may include an air/fuel inlet (or mouth) where wood fuel or similar biomass is fed into the mouth of the stove, and air is drawn into the mouth through convection. In these embodiments, combustion typically occurs within a combustion chamber. The combustion chamber's geometry and materials may be optimized for proper combustion of the biomass fuel and to minimize heat transfer to the stove body. In many embodiments, exhaust gas from combustion of the biomass fuel may be drawn up through an upper combustion chamber and into one or more exhaust inlet orifices. In some embodiments the exhaust inlet orifices are defined in a ring structure positioned at or near the top of the upper combustion chamber. This ring structure may be referred to as an “EGR inlet skirt,” and its interior may define an exhaust collection chamber. One embodiment of the “EGR inlet skirt” is pictured in FIG. 7. A pump/blower device may be integrated into the stove to aid in drawing the exhaust gas into the EGR inlet skirt. The exhaust gas travels through the conduit and passes through the pump/blower device and into one or more injection conduits until it is injected into the combustion chamber of the stove.

In many embodiments, exhaust gases may enter an intake orifice that may be located at or near the top of the combustion chamber. The orifice may be in fluid communication with an interior of an exhaust collection chamber, the chamber being in fluid communication with one or more exhaust conduits that contain the exhaust and channel it to a pump or blower in fluid communication with the exhaust conduit. The pump or blower aids in actively moving the exhaust from the exhaust conduit into one or more injection conduits, which channel the exhaust into one or more injection nozzles. The injection nozzles having a plurality of orifices that allow the exhaust to escape the interior of the injection nozzles.

Testing of the disclosed device and systems indicated that the injection location, nozzle geometry and flow rate affect emissions performance of the stove. Thus, optimized flow rates were determined for various combinations of injection location and nozzle geometry. Next, minimized PM emissions for various combinations were compared to determine an optimized design for the test stove. Some preferable combination embodiments, while not entirely optimized for injection location and nozzle geometry, were identified by the disclosed testing. Some exemplary combinations are described below.

Example Stove Nozzle Design

There are several locations for placing the injection nozzle within the combustion chamber. Likewise, several geometries and configurations are possible for the injection orifices defined in the surface of the nozzle. Testing was performed on various designs, and resulted in a preferred injection location and nozzle configuration for PM emissions reductions for one embodiment of the disclosed stove. This preferred embodiment of injection location and nozzle geometry is pictured in FIGS. 11 and 12. In this embodiment, the injection nozzle location is at or near the top of the combustion chamber.

In this embodiment, there are two injection nozzles positioned at or near the sides of the combustion chamber. Other embodiments may include more than two nozzles or one nozzle. In this embodiment, the nozzles are positioned horizontal and perpendicular to the direction of the draft of combustion products through the upper combustion chamber of the stove. In some embodiments, the nozzle(s) may be other than horizontal or perpendicular to the draft. In this embodiment, 6 injection orifices are defined within the surface of the nozzle(s). In this embodiment, the injection orifices are spaced at 9/16 inch center to center, with each orifice having a diameter of approximately 3/16 inch. In many embodiments, for example embodiments wherein the nozzle defines a substantially linear tube structure, such as the embodiment shown in FIGS. 12 and 16, the first orifice (the orifice positioned closest to the mouth of the combustion chamber) is located approximately ½ inch from the combustion chamber mouth. In other embodiments, the nozzle can define a ring structure that may be positioned at or near the wall of a upper chamber (or chimney).

The injection nozzle(s) may define injection orifices that are confined to a horizontal section of the combustion chamber. In many embodiments, the horizontal section is less than about 20 cm, 19 cm, 18 cm, 17 cm, 16 cm, 15 cm, 14 cm, 13 cm, 12 cm, 11 cm, 10 cm, 9 cm, 8 cm, 7 cm, 6 cm, 5 cm, 4 cm, 3 cm, 2 cm, or 1 cm, and greater than about 0.5 cm, 1 cm, 2 cm, 3 cm, 4 cm, 5 cm, 6 cm, 7 cm, 8 cm, 9 cm, 10 cm, 11 cm, 12 cm, 13 cm, 14 cm, 15 cm, 16 cm, 17 cm, 18 cm, or 19 cm. In some embodiments, such as that shown in FIGS. 16, 26, and 29, the orifices are positioned substantially planar within the combustion chamber and parallel the floor of the combustion chamber. Where the outlet orifices are planar to the floor, the center of each outlet orifice is positioned at the same distance from the floor of the combustion chamber (or the distances vary less than about 0.5 cm). In some embodiments the distances from the floor to each orifice varies by less than 0.5 cm, 1 cm, 2 cm, 3 cm, 4 cm, 5 cm, 6 cm, 7 cm, 8 cm, 9 cm, 10 cm, 11 cm, 12 cm, 13 cm, 14 cm, 15 cm, 16 cm, 17 cm, 18 cm, or 19 cm (where the variance is less than about 0.5 cm, the orifices may be said to be planar). Where the nozzle defines a ring structure, the orifices may be positioned throughout the ring and designed to direct gas into the center of the ring.

In many embodiments, a preferred flow rate may exist for the flow of exhaust into the combustion chamber that may aid in reducing PM emission from the stove fitted with the disclosed system. In many embodiments, the preferred flow rate may be from about 20 to 70 standard liters per minute (SLPM). For the nozzle configuration described above and shown in FIGS. 11 and 12, the preferred flow rate for PM emission reduction varies from 50 to 70 standard liters per minute. In most embodiments, the flow rate may increase with increasing firepower to minimize PM emission. For example, at a firepower of 2.4 kW the preferred flow rate may be about 50 SLPM, and at a firepower greater than 2.6 kW the flow rate may increase to 70 SLPM to minimize PM emission. In some cases, firepowers less than approximately 2.4 kW, the flow rate may be required to be adjusted below 50 SLPM to avoid blowing out the flame, for example less than about 40 SLPM, 30 SLPM, 20 SLPM, 10 SLPM, or 5 SLPM. In many embodiments, the flow rate of the gas is correlated to the stove's firepower. For example, obtaining desired PM reductions in a 2.5 kW stove may require a flow rate of approximately 40 SLPM, while a 20kW stove (e.g. a chimney-drafted stove) may require approximately 60 SLPM for the same level of PM reduction. In some embodiments the flow-rate may be less than about 110 SLPM, 100 SLPM, 90 SLPM, 85 SLPM, 80 SLPM, 75 SLPM, 70 SLPM, 65 SLPM, 60 SLPM, 55 SLPM, 50 SLPM, 45 SLPM, 40 SLPM, 35 SLPM, 30 SLPM, 25 SLPM, or 20 SLPM, and greater than about 10 SLPM, 20 SLPM, 25 SLPM, 30 SLPM, 35 SLPM, 40 SLPM, 45 SLPM, 50 SLPM, 55 SLPM, 60 SLPM, 65 SLPM, 70 SLPM, 75 SLPM, 80 SLPM, 85 SLPM, 90 SLPM, 100 SLPM, or110 SLPM.

The embodiment of an EGR-enabled stove depicted in FIGS. 11 and 12 was able to reduce PM₁₀ emissions for a cold-start water boil test from 275 mg/MJd (milligrams PM₁₀ per Mega joule delivered to the water) for the baseline M5000 stove with the pot skirt, to 125 mg/MJd for the same stove with the optimized EGR flow rates.

To better understand the variables affecting PM emissions reduction for the embodiment depicted in FIGS. 11 and 12, some of the potential variables were analyzed experimentally. These experiments suggest that one mechanism for reducing PM mass emissions was the increased size of the oxidation region due to elevated O₂ concentrations within the flame, which in turn increases the fuel's access to oxidizer. The oxidation region is the area within the flame where oxygen has diffused in concentrations high enough to support combustion. Furthermore, these tests showed that when O₂ was injected closer to the start of the oxidation region (closer towards the lowest point of the flame front), it was more effective at reducing particulate mass emissions. In contrast, injection directed toward the charcoal base or directly into or onto the fuel was found to result in higher PM mass emissions.

The embodiments depicted in FIGS. 11 and 12 use a nozzle location (near the top of the combustion chamber) that allowed injecting the gas near the bottom of the oxidation region but sufficiently above the fuel to prevent smoldering or blow out of the flame and consequently higher emissions.

These studies also identified other variables that may affect PM emissions. For example, increased residence time of particles in the oxidation zone (such as via recirculation) helped to decrease PM emissions, as did forced mixing of the combustion gasses. However, in many cases, the effect from manipulating these two variables appeared to be less than the effect of elevated O₂ concentration in the oxidation region of the flame. Additionally, the isolated effect of CO₂ recirculation was determined to have less effect on PM emissions than elevating O2 concentrations. In some cases, an increase in fuel consumption rate was observed in some embodiments of the disclosed EGR-enabled stoves, which may, in isolation, result in an increase in PM emissions. However, when these effects are combined, as they are in many of the disclosed EGR-enabled systems, a net reduction in PM mass emitted is observed.

Because testing suggested that elevated O₂ concentration in the oxidation region of the flame was one mechanism for PM mass reduction, the same nozzle configuration and injection location was tested using fresh air (non-EGR) injection, which should also provide higher O₂ concentrations. With complete fresh air injection, the cold-start water boil test PM₁₀ emissions were reduced to 91 mg/MJd. This suggested that an EGR embodiment with this nozzle geometry and injection location may provide reductions in PM emissions greater than or equal to those using exhaust gas.

As described below, the disclosed EGR systems and devices may be modified in various way to reduce emissions from a biomass combustion stove. For example, fresh air injection, nozzle location and nozzle geometry can be modified. Other modifications, variations, and permutations of the disclosed systems and devices may be included to further reduce PM mass and these are included within the scope of the present disclosure.

All references disclosed herein, whether patent or non-patent, are hereby incorporated by reference as if each was included at its citation, in its entirety. In case of conflict between reference and specification, the present specification, including any definitions, will control.

Although the present disclosure has been described with a certain degree of particularity, it is understood the disclosure has been made by way of example, and changes in detail or structure may be made without departing from the spirit of the disclosure as defined in the appended claims.

EXAMPLE II EGR—Analysis of PM Emissions Reductions

The embodiment of FIG. 7 was used to perform initial testing of exhaust gas recirculation. Specifically, particulate matter (PM_(2.5)) was measured during these tests to test for effective emissions reduction. The tests analyzed several variables—for example: Temperature of recirculated exhaust gas, Exhaust gas composition, Exhaust gas injection location and nozzle configuration, and Exhaust gas flow rate.

Recirculated Gas Composition

To ensure that the amount of ambient air drawn into the EGR pot skirt was minimized, two sets of water boiling tests (WBTs) were run with consistent EGR flow rates, injection locations and nozzle configurations, but with different configurations of the inlets: one set was performed with a ring device as described above, and a second set of data was collected using a conduit that drew exhaust directly from the center of the combustion chamber outlet. This second configuration ensured that all or nearly all gas drawn into the system comprised exhaust gas. The concentration of molecular oxygen was then sampled throughout the tests using a gas analyzer (TESTO) positioned within the path of the EGR system. By comparing the oxygen concentration it could be determined if ambient air (fresh air or non-exhaust gas) was entering the system at high flow rates.

Temperature of Re-Circulated Gas

The temperature of the recirculated exhaust gas was tested by incorporating a heater tape with a proportional-integral-derivative (PID) temperature controller into the testing platform. A picture of this testing system is seen in FIG. 13.

10 cold-start water boiling tests were completed, in which test variables were held constant except for the temperature of the recirculated gas. The average recirculated gas temperature was varied from approximately 20° C. to 100° C. The lower limit of tested temperatures replicated the effect of exhaust gas that had been entirely cooled to ambient temperatures. The upper bound of tested temperatures represented a scenario in which the exhaust gas temperatures at the lower edge of the pot were maintained nearly constant throughout the system, from intake from the exhaust to injection into the combustion chamber. The gas temperature was cooled through the use of an ice bath to lower the gas temperature and an electrical resistance heater tape was used to raise gas temperatures. Thermocouples along the EGR path were used to measure temperature of the gas at one second intervals.

Nozzle and Flow Rate Optimization Procedure

These studies were also used to compare the minimized PM_(2.5) emissions for various nozzle configurations. A unique nozzle and flow rate optimization procedure was developed for rough approximation of minimized PM_(2.5) emissions for a particular nozzle configuration. Two assumptions are made in these tests discussed further below. In general progression of the test procedure includes Step 1—designing and fabricating the nozzle, Step 2—determining flow rate for start up firepower phase, Step 3—determining optimized flow rate for steady state firepower phase, and Step 4—running cold start water boil tests at optimized flow rates.

In the EGR Experimental Optimization portion of this study, 3 iterations of this procedure were completed. The details of each step in a sample iteration of the procedure are discussed below. Step 1: A nozzle is designed and fabricated. The nozzle is designed using a combination of fluid mechanics and combustion concepts and previous experimental results. Step 2: In step 2 the optimized flow rate for PM_(2.5) emissions reductions for the start-up phase is determined. For each data point, the standard cold-start water boil test procedure was followed but was concluded at a water temperature of 30° C. A water temperature of 30° C. was selected to be the ending value for these tests. When the water temperature reaches 30° C. the firepower is no longer in a transient phase and has entered a steady state. The flow rate was altered at 10 to 20 SLPM intervals, with 1 to 2 data points collected per flow rate. Sample results of a start-up phase flow rate optimization can be seen in the chart in FIG. 14. This chart shows that an optimized start-up phase flow rate for this example data set is approximately 40 SLPM.

In Step 3 the optimized flow rate for PM_(2.5) emissions reductions for the steady state firepower phase is determined. A quasi-simmer test approach was utilized for the determination of the steady state optimized flow rate. Firstly, the stove body, pot and water were brought up to simmer temperatures. Once steady state simmer temperatures were achieved, the charcoal and burning fuel were removed while the pot and water were left in place. A known weight of fuel was then reignited in the combustion chamber with a propane torch. A propane torch was used to reignite the fuel because the steady state firepower phase (3¾″×¾″×12″ pine sticks) fuel feeding approach was used from the start of each sample. The fire was allowed to burn for one minute before sampling of PM_(2.5) began. This one minute delay prevented sampling PM_(2.5) produced during the ignition of the sticks. The PM_(2.5) emissions were then sampled for ten minutes, or until the sticks were nearly consumed, while the firepower was held at a constant level. Upon completion of the PM_(2.5) sampling period the leftover fuel and charcoal were removed and weighed. The re-ignition process and subsequent ten-minute sampling period was then repeated, but at a different flow rate. Throughout the process of this test the water and pot remained in place and were kept at boiling temperatures. Additionally, the time between sampling periods was less than 3 minutes, which prevented significant cooling of the stove and/or pot. This test procedure allowed for a rough approximation to be developed of the relationship between PM emissions and flow rate for a particular nozzle setup during the steady-state firepower phase. The PM emissions were characterized in terms of mass of PM_(2.5) emitted per mass of fuel consumed, referred to as the Emissions Factor (EF). In the calculation of the EF, the weight of fuel consumed was corrected for moisture content and for sample time. Example results of a steady-state firepower phase flow rate optimization can be seen in FIG. 15. It can be seen that the example optimized steady-state phase flow rate is approximately 80 SLPM.

In Step 4, the minimized PM emissions for a particular nozzle were determined. Using the optimized flow rates determined in Steps 2 and 3, cold-start water boil tests were completed. The optimized flow rate determined in step 2 would be used until a water temperature of about 30° Celsius and the optimized flow rate determined in step 3 would be used for the remaining time of the cold start. The PM emissions resulting from step 4, along with supplementary conclusions derived from steps 2 and 3, were then used in the design of the next nozzle.

Several assumptions were made during these tests. Specifically, in these test procedures, the cold start water boil test is split in to two phases, the start-up phase and the steady-state phase. Testing concluded that emissions during the start-up phase, where there is a more transient and generally lower firepower, and emissions during the steady state firepower phase, where there is a steady and generally higher firepower, are different. Thus, it was hypothesized that different firepowers would require different forced draft flow rates to achieve maximal PM emissions reductions. Consequently, the cold start water boil test emissions may be minimized if the flow rate tracks the real-time firepower. For the flow rate to follow the real-time firepower continuously, a function relating firepower and flow rate can be developed. This function allows determination of optimized flow rates for numerous firepowers, and a control system that continuously monitors firepower. Alternativley, the controller may operate on a time-based function (step, block, or continuous).

In order to save time, optimized flow rates were determined for only two distinct firepower phases and the flow rates during the cold start tests followed a step function pattern. The assumption inherent in this method is that the measured PM_(2.5) emissions for a cold start test using the two firepower phase model will be similar to emissions from a cold start test in which the flow rate continuously follows real-time firepower.

The flow rate of gas injection may vary depending on firepower production. In some embodiments, a fan placed in the exhaust path may aid in regulating flow rate based upon fire power. In other embodiments, the flow rate may be held constant during fan operation. In further embodiments, the fan may operate as a multi-step speed, wherein there are two or more operation speeds. In many embodiments, a controller may be programed with various fan speed functions based on firepower (which may be measure) or time of use.

Use of flow rate determined by the steady-state flow rate optimization procedure may, in some cases, rely on an assumption that the draft flow rate does not significantly affect the thermal efficiency of the stove during the steady state firepower phase. This assumption was made because it was not practical in these tests to take accurate measurements of the energy delivered to the water during this test.

Nozzles Tested

Ultimately, three iterations of this optimization procedure were completed for the EGR stove. The three different nozzle configurations that were tested are seen in FIG. 16. These setups occupy two major injection locations. The approximate injection locations are displayed below in a cross-sectional depiction at the lower right of FIG. 16. A diffusion nozzle is designed to inject gas below the fuel bed and is positioned below the fuel. An air curtain nozzle is designed to inject gas at or near the mouth or inlet and at the top of the combustion chamber. Side injection nozzles are also designed to inject gas at or near the top of the combustion chamber. The air curtain embodiment shown in FIG. 16 has approximately a 4″×¼″ wide gap and injects gas downwards at an angle of about 45° from horizontal. The side injection nozzles shown in the embodiment of FIG. 16 have 4.9 millimeter diameter holes that inject air perpendicularly to the natural draft of the stove. The diffusion nozzle embodiment of FIG. 16 has two ¾′ diameter pipes that are flattened and direct gas beneath a perforated metal grate.

Recirculated Gas Composition—Results/Discussion

The composition of the recirculated gas can have a significant effect on the concentrations of nitrogen, oxygen, carbon dioxide and carbon monoxide within the combustion chamber. As discussed above, at high EGR flow rates ambient air (fresh air or non-exhaust gas) could be drawn into the EGR pot skirt inlet affecting test results. Average oxygen concentrations using the two inlet configurations described above are shown in Table 1, below.

TABLE 1 Recirculated Exhaust Gas Composition Average Test O₂ Concentration PM Emissions Number Testing Platform in Recirculated Gas (%) (mg/MJd) 1 Standard 15.9 260 2 Standard 16.1 390 3 Modified EGR inlet 16.4 320 4 Modified EGR inlet 16.9 250

Table 1 results demonstrate that the average oxygen concentration in the recirculated exhaust gas appears to increase slightly when the modified EGR inlet (that takes exhaust directly from the center of the combustion chamber outlet) is installed. However, if ambient air was being drawn into the standard EGR pot skirt inlet, then the oxygen concentration in the recirculated exhaust gas would be expected to be higher than with the modified EGR inlet installed. This effect was not observed, indicating that little or no ambient air is drawn into the standard EGR inlet at high EGR flow rates. The fact that the opposite relationship is observed may instead indicate that when the modified EGR inlet is installed, the gas is drawn in before combustion of available O₂ is complete.

Temperature of Recirculated Gas

The effect of temperature of the recirculated gas was tested through a set of eight cold start WBTs. Four different gas path setpoint temperatures were tested, with two replicates for each temperature. The results of these tests are pictured in FIG. 17. The temperature value plotted on the x-axis represents the average measured temperature of the gas throughout a WBT, measured immediately before injection into the combustion chamber. It should be noted that for all tests, the Air Curtain style nozzle was used, and the EGR flow rates were kept consistently at approximately 70 SLPM.

Results from these experiments, shown in FIG. 17, did not identify a strong correlation between gas temperature and PM emissions. While some embodiments may include insulation positioned in the gas path, many embodiments may not include an insulated gas path. For subsequent testing, described below, insulation along the EGR path was not incorporated.

Nozzle and Flow Rate Optimization

Results of crude flow rate optimizations for the air curtain, diffusion, and side injection nozzle are displayed in FIGS. 18, 19 and 20, respectively. Table 2 summarizes the major results of the flow rate optimizations. The values for the optimized flow rate were determined by selecting the approximate minimum location on the plots in FIG. 18. The values for PM_(2.5) emissions in Table 2 represent the phase specific emissions at the optimized EGR flow rates.

TABLE 2 EGR Nozzle Flow Rate Optimization Diffusion Air Side Injection Phase Nozzle Set Up Nozzle Curtain Nozzles Start-up Optimized Flow (SLPM) 110 40 50 PM (mg/MJ_(d)) 420 310 260 PM Emissions Factor 2.2 1.3 1.1 (mg/g) Steady- Optimized Flow (SLPM) 60 0 70 State PM Emission Factor 0.64 0.98 0.57 (mg/g)

Using the optimized flow rates seen in Table 2, cold start WBTs were completed for each nozzle configuration. The results of these tests can be seen in Table 3-1. The PM_(2.5) emissions for these tests represent the PM_(2.5) emission minimization for each nozzle configuration. It can be seen in Table 3 that the Side Injection Nozzles, with injection near the top of the combustion chamber, led to the greatest reduction in PM_(2.5) emissions, with a 44% reduction from the baseline M5000 PM_(2.5) emissions (See Table 3-2 for M5000 baseline values; PM; 461 mg baseline vs 248 mg for side injection nozzles; PM per Energy 280 mg/MJd baseline vs 150 mg/MJd for side injection nozzles). Use of side injection nozzles also appear to increase the firepower, compared to typical baseline firepower and the other two nozzle configuration's firepower.

TABLE 3-1 Minimized PM_(2.5) Emissions for Various Nozzles Using EGR Air Side Nozzle Type Curtain Diffusion Injection Number of Tests 1 1 2 Time to Boil (min) 32.4 40.8 28.9 Temperature Corrected Time to Boil 32.1 40.2 32.1 (min) Dry Fuel Consumed (g) 280 282 317 Average FP (kW) 2.4 2.0 2.9 Total Thermal Efficiency (%) 36 35 32 PM (mg) 294 334 248 PM per Energy Delivered (mg/MJd) 180 200 150 CO (g/MJd) — — 2.7* one test

TABLE 3-2 Baseline M5000 Performance 80% Sample Confidence IWA Mean Interval Tier Time to Boil (min) 28.9 27.0-30.9 N/A Temperature Corrected Time to Boil 28.7 26.8-30.5 N/A (min) Dry Fuel Consumed (g) 286 275-297 N/A Char Produced (g) 17 15-18 N/A Average FP (kW) 2.9 2.6-3.2 N/A Total Thermal Efficiency (%) 36 35-37 Tier 3 PM (mg) 461 331-592 N/A PM per Energy Delivered (mg/MJd) 280 210-340 Tier 2 CO (g/MJd) 2.7 2.6-2.8 Tier 4

Using the emissions factors found in Table 2, and with knowledge of the total amount of fuel consumed in a WBT, the total weight of emitted PM can be predicted. Comparison between the predicted PM and measured PM allows one to determine whether the measurements in the flow rate tests can provide a relatively accurate representation for emissions from a full cold start WBT. The comparison between predicted and measured PM emissions can be seen in Table 4.

TABLE 4 Predicted versus Measured PM_(2.5) Emissions Side Diffusion Air Injection Nozzle Setup Nozzle Curtain Nozzles Total Dry Weight Fuel Use to Boil (g) 282 280 317 Dry Weight Fuel Use in Start-up Phase 66 75 80 (g) Dry Weight Fuel Use in Steady-state 220 210 240 Phase (g) Predicted PM Start-up Phase (mg) 147 101 90 Predicted PM Steady-state Phase 138 185 135 (mg) Total Predicted PM (mg) 285 286 225 Measured PM (mg) 334 294 248 Error (%) 14.6% 2.9% 9.3%

The error values seen in Table 4 indicate that the shorter tests used in the flow rate optimization can be used as predictors of the total optimized cold start emissions with accuracy. Additionally, this indicates that the constant efficiency assumption in step 3 of the nozzle optimization procedure does not introduce a significant inaccuracy.

Ultimately, the results of this section indicate that an emissions reduction can be achieved through the application of an EGR system to an existing stove, such as the Envirofit M5000 stove. The tests also provide support for the use of the four-step procedure described above to create and test new nozzle configurations.

EXAMPLE III Analysis of Variables Involved in EGR Emissions Reductions

In the tests described in the previous section, it was shown that emissions reduction can be achieved through the application of EGR. The present section uses similar tests to identify, isolate, and measure various mechanisms affecting emissions reduction observed with EGR through the side injection nozzles.

Testing Platform

The M5000 was again used for these tests, however, the EGR stove testing platform was modified from that described above. First, conduit was routed such that gas could be injected into the M5000 from compressed gas cylinders. The flow rate of the injected gases was regulated through a high performance Alicat Mass Flow Controller.

Testing Methodology

It was hypothesized that the reduction in PM emissions with the EGR cookstove could be the net result of a combination of the following mechanisms:

Increased particle residence time;

Chemical Effects of O₂/CO₂;

Mixing;

Dilution;

Temperature; and

Firepower.

The present tests were performed in order to isolate and determine, as best as possible, the relative magnitude of each of these mechanisms with reference to the EGR side injection nozzle configuration.

Increased Particle Residence Time

The effect of increased residence time of particulate material in the flame was determined by injecting a particulate-free replicate EGR gas into the stove. The EGR replicate gas was composed of 15% O₂, 5% CO₂ and 80% N₂, similar to the composition measured above. The replicate gas was injected at an equivalent mass flow rate to the optimized mass flow rates used for the Side Injection Nozzles. In experiments described above, it was determined that the optimized flow rates for the Side Injection Nozzles are 50 to 70 SLPM for the start-up and steady-state firepower phases, respectively. However, measurement of the temperature at the blower wheel throughout the cold WBTs at the optimized EGR flow rates for the Side Injection Nozzles indicated that the average temperature at the fan was approximately 333 K. In order to match mass flow rate of the EGR gas with the replicate EGR gas a temperature correction was applied and flow rates of 44.7 to 62.6 SLPM were used (where SLPM is according to Alicat Mass Flow controller specs, 298 K and 14.7 psi). In total, three cold start WBT replicates were completed using the EGR replicate gas.

Chemicals Effects of CO₂/O₂ Recirculation

In order to replicate/isolate the chemical effect of O₂, pure molecular O₂ was injected into the stove using the Side Injection Nozzle configuration. Two important considerations must be made in order to approximate the chemical effect of O₂ seen in the EGR stove tests previously described. One, the mass flow rate of pure injected O₂ should be similar to the mass flow rate of O₂ injected in the optimized EGR stove test. Considering that the EGR gas was comprised of approximately 15% O₂, a flow rate equivalent to 15% of the temperature corrected optimized flow rates should be used. This leads to flow rates for pure O₂ injection of 6.7 and 9.4 SLPM for the start-up and steady-state firepower phases, respectively. Second, the velocity of the injected O₂ should be similar to the velocity of the injected EGR gas in order to simulate the injection depth of the gas into the combustion chamber. In order to match the velocities between tests, the diameter of the holes through which gas escapes in the Side Injection Nozzles was modified such that the new hole area was reduced to 15% of the original hole area. Three cold start WBT replicates were completed in which pure O₂ was injected into the stove.

In order to replicate and isolate the chemical effect of CO₂, pure CO₂ was injected into the stove. Because CO₂ comprises approximately 5% of the EGR gas, a flow rate equivalent to 5% of the temperature corrected optimized flow rates should be used. This leads to flow rates for pure CO₂ injection of about 2.2 to 3.1 SLPM for the start-up and steady-state firepower phases, respectively. Because these flow rates are significantly lower than the total draft through the stove, matching the injection depth with the Side Injection Nozzles was difficult. To accommodate this issue the Diffusion Nozzle setup was used. This ensured that the natural draft of the stove carried the CO₂ into the combustion chamber in a central location of the flame. Three WBT replicates were completed.

Because the CO₂ flow rates that mimic EGR gas comprised of 5% CO₂ were extremely low, additional tests were run with higher CO₂ flow rates to make any chemical/physical effects more evident. The same procedure as used in the pure O₂ injection tests was followed, instead replicating the chemical effect of CO₂ in EGR gas that is comprised of 15% CO₂. Three WBT replicates were completed.

Mixing, Dilution and Temperature Reduction Due to Recirculated Nitrogen

To complete this study, Argon was injected at the optimized EGR flow rates through the side injection nozzles. In total, three cold start WBTs were performed.

Initially, it was attempted to run these tests using pure nitrogen, replicating exactly the mixing, dilution and temperature reduction effects of recirculated nitrogen. However the heat capacity of the nitrogen made it impossible to maintain flame. Thus argon was selected for these tests because it has half the heat capacity of nitrogen. Argon's low heat capacity allows for a better understanding of the effects of cooling of the flame, without cooling the flame so much that combustion cannot be maintained.

Additionally, its inert nature allows for the isolation of mixing, dilution and cooling effects by taking intrinsic chemical effects out of the equation.

Firepower

In order to isolate the effect of increased firepower observed with the EGR tests in Section 4.4.3, the expected PM emissions at the measured firepower in the EGR stove are compared with the baseline M5000 emissions using the same fuel feeding approach.

Additionally, all tests completed in this section were compared to the expected baseline emissions based on measured firepower, using the relationship between Firepower and PM emissions in the baseline stove derived above. This allowed for a more accurate isolation of the effects of each tested mechanisms.

Results/Discussion

All tests were completed using the Side Injection Nozzle configuration and referenced the optimized flow rates of 50 to 70 SLPM determined above. Thus, the magnitude of each mechanism's effect on PM emissions should be compared to the PM emissions reduction observed in the EGR tests with the Side Injection Nozzles, seen in Table 3-1. As a reminder to the reader, the average cold start water boil test PM emissions achieved with this optimized EGR configuration was 150 mg/MJd.

Increased Particle Residence Time

In the EGR stove a portion of the particulate matter is recirculated through the flame. This will results in a decrease of the net PM_(2.5) mass emitted, assuming that oxidation is the net dominating mechanism along the particle's path. As indicated above, formation and oxidation rates begin to compete at temperatures of approximately 800° C. Fortunately, temperature profiles measured in a rocket elbow stove exceed 800° C. in a large portion of the combustion chamber. This implies that oxidation may exceed formation rates and that particle recirculation through the flame may play a large role in reducing PM_(2.5) emissions.

The effect of recirculating the PM_(2.5) and consequently increasing its oxidation time was isolated by performing a set of cold start WBTs that used particle-free EGR replicate gas, composed of 80% N₂, 15% O₂ and 5% CO₂. The results of these tests can be seen in Table 5, below.

TABLE 5 EGR Replicate Gas Test Results Test Description EGR Replicate Gas Injection Number of Tests Completed 3 Mean 80% CI Time to Boil (min) 24.1 22.4-25.9 Temperature Corrected Time to Boil (min) 24.0 22.4-25.6 Fuel Use to Boil (g) 308 301-316 FP kW 3.4 3.2-3.6 CS Thermal Efficiency (%) 33 32-34 PM (mg) 294 205-384 PM per Energy Delivered (mg/MJd) 180 130-240

In tests described above, it was determined that firepower plays a large role in PM emissions, and a model for the baseline M5000 PM_(2.5) emissions was proposed. In order to better isolate the effect of particle recirculation, the tests detailed in Table 5 are compared to the expected baseline M5000 emissions based off of the measured firepower for these tests. This allows for the comparison of emissions produced from the tests in Table 5 to baseline emissions at the same firepower, taking firepower out of the equation and allowing for more accurate experimental isolation of the mechanism of interest. This comparison is detailed in Table 6.

TABLE 6 EGR Replicate Test Results Compared to Firepower Corrected Baseline Average PM_(2.5) (mg/MJ_(d)) for EGR Replicate Gas Tests 180 Average Measured FP (kW) 3.4 Predicted Baseline PM_(2.5) (mg/MJ_(d)) at Measured FP 440

It can be seen in Table 6 that a reduction from 440 (mg/MJd) to 180 (mg/MJd) was achieved using particle-free EGR replicate gas. This indicates that a large portion of the PM_(2.5) emissions reduction observed in the EGR stove was not caused by particle recirculation, but was instead caused by the chemical and physical effects of the gaseous component of the recirculated exhaust products. However, the resulting emissions for the particle free EGR replicate gas tests are slightly higher than the optimized EGR stove emissions of 150 (mg/MJd). This may imply that particle recirculation accounted for some reduction in emissions from 180 (mg/MJd) to 150 (mg/MJd).

Chemical Effects of CO₂/O₂ Recirculation

Previous measurements of the recirculated gas indicated that it is composed of approximately 15% O₂, 5% CO₂, and 80% N₂ on average during the optimized EGR stove cold start WBTs. Other gaseous constituents may include carbon monoxide and argon but their concentrations were sufficiently low to deem their effects negligible. Given that N₂ acts as a relatively inert gas due to the relatively low combustion temperatures experienced during biomass combustion in rocket elbow stoves, it can be assumed that the major chemical effects may result from O₂ and CO₂ recirculation.

The results of the tests to isolate the chemical effects of O₂ and CO₂ can be seen in Table 7. Note, that the “pure O₂ injection at 15% of the optimized flow rate” and “pure CO₂ injection at 5% of the optimized flow” tests directly replicate their respective chemical effects in EGR gas that is composed of 15% O₂ and 5% CO₂. The “pure CO₂ injection at 15% of the optimized flow” tests were completed in order to make any potential chemical effects of CO₂ more evident. Given that the recirculated gas must always be 80% N2, a 15% CO₂ composition would indicate that the stove is operating near with near stoichiometric levels of total draft. Because rocket elbow stoves typically operate with significantly fuel lean concentrations, then replicating a 15% CO₂ composition can be assumed to be an estimate for the absolute maximum potential chemical effect of CO₂.

TABLE 7 Effects of CO₂ and O₂ Test Results Test Description Pure O₂ Injection 15% Pure CO₂ Injection, 5% Pure CO₂ Injection, 15% of Optimized Flow of Optimized Flow of Optimized Flow Number of 3 3 3 Tests Completed Mean 80% CI Mean 80% CI Mean 80% CI Time to Boil (min) 25.2 24.2-26.2 29.3 27.7-30.9 26.7 25.9-27.6 Temperature Corrected 25.0 24.0-26.1 29.2 27.5-30.8 26.7 25.6-27.7 Time to Boil (min) Fuel Use to Boil (g) 280 271-288 287 285-290 297 289-304 FP kW 3.0 2.8-3.2 2.5 2.3-2.7 2.9 2.9-3.0 CS Thermal Efficiency 36 35-37 37 35-39 36 35-36 (%) PM (mg) 167   147-186.4 523 467-578 540 387-692 PM per Energy 100  90-110 320 290-360 320 240-410 Delivered (mg/MJd)

The test results detailed Table 7 are compared in Table 8 to the expected baseline M5000 emissions based off of the measured firepower for these tests. This allows for a more accurate isolation of the chemical effects of interest, by taking firepower variations out of the equation.

TABLE 8 Effects of CO₂ and O₂ Test Results Compared to Firepower Corrected Baseline Pure O₂ Pure CO₂ Pure CO₂ Injection Injection, Injection, 15% of 5% of 15% of Optimized Optimized Optimized Test Description Flow Flow Flow Average PM_(2.5) 100 320 320 (mg/MJ_(d)) Average FP (kw) 3.0 2.5 2.9 Average FP 260 300 320 Corrected Baseline PM_(2.5) (mg/MJ_(d)) Percent Reduction 61% −8% −2% in PM_(2.5) from Predicted Baseline

It can be seen in Table 8 that pure O₂ injection leads to a dramatic reduction in mass of PM emitted. The visually observed mixing effect throughout these tests was negligible, and it can be assumed that the O₂ concentration near the fuel was unaffected. However, it was visually observed that the O₂ was injected into the flame near the top of the lower combustion chamber, such that the streams of injected O₂ on either side of the combustion chamber converged as they entered the chimney of the M5000. Thus, it can be concluded that a significant reduction in PM_(2.5) mass occurred and was due primarily to a chemical effect caused by elevating the concentrations of O₂ within the region of the flame above the fuel.

The isolated chemical effects of CO₂ do not appear to cause any significant effect on the mass of PM_(2.5) emitted. This indicates that the chemical effect of CO₂ across the range of potential concentrations has an insignificant effect on PM emissions.

Mixing, Dilution and Temperature Reduction

In order to better understand the various effects of recirculating nitrogen through the Side Injection Nozzles, Argon was injected at the optimized EGR stove flow rates. Three cold start WBT's were completed using Argon injection at the optimized flow rates, the results of which can be seen below in Table 9.

TABLE 9 Effects of Mixing, Dilution and Temperature Reduction Test Results Test Description Argon Injection at Optimized Flows Number of Tests Completed 3 Mean 80% CI Time to Boil (min) 26.1 24.3-27.9 Temperature Corrected Time to Boil (min) 26.3 24.4-28.2 Fuel Use to Boil (g) 317 315-319 FP kW 3.1 2.9-3.3 CS Thermal Efficiency (%) 34 33-34 PM (mg) 618 504-733 PM per Energy Delivered (mg/MJd) 380 310-450

As described in previous sections, the tests results detailed Table 9 are compared in Table 10 to the expected baseline M5000 emissions based off of the measured firepower for these tests. This allows for a more accurate isolation of the mechanisms of interest by taking firepower variations out of the equation.

TABLE 10 Effects of Mixing, Dilution and Temperature Reduction Test Results Compared to Firepower Corrected Baseline Test Description Argon Injection Average PM_(2.5) (mg/MJ_(d)) 380 Average FP (kw) 3.1 Average Expected Baseline PM_(2.5) (mg/MJ_(d)) 340 Percent Reduction in PM_(2.5) from Predicted Baseline −10%

It can be seen in Table 10 that the effect of injecting Argon caused a minor increase in PM_(2.5) emissions. This minor increase in emissions is the net result of a combination of enhanced mixing, reactive constituent dilution and temperature reduction.

In order to better understand the individual effects of these three mechanisms, we will first consider mixing. For this test set, the optimized EGR stove mass flow rates were matched with Argon. Every recirculated exhaust gas molecule was replaced with an Argon molecule that injected at the same velocity. Consider that mixing is a function of the particle momentum, and consider that Argon's molecular weight is 40 (kg/kmole) whereas the recirculated exhaust gas molecular weight is approximately 29 (kg/kmole). Because Argon's molecular weight is larger, its momentum and mixing effect will be approximately 38% larger.

The effect of enhanced mixing in the combustion processes of solid biomass fuels has been documented to reduce emissions of carbonaceous particles. Emissions of soot and soot precursors are compounded by poor mixing where pockets of unburned vapors and particles may exit the combustion zone. Thus, it is assumed that the isolated effect of the mixing caused by the Argon reduces PM_(2.5) emissions.

However, the total effect of Argon injection actually led to an increase in PM_(2.5) emissions. Thus, Argon's combined effects of dilution and temperature reduction actually caused an increase in PM_(2.5) emissions. In order to better understand the effect of cooling on the flame, a comparison is made between the cooling capacity of the recirculated nitrogen in the optimized EGR stove, and the cooling capacity of the injected Argon. The heat capacity of Argon is 0.52 (kJ/(kg-K)) and the heat capacity of N2 is 1.04 (kJ/(kg-K)). Additionally, the calculated mass flow rate during the steady state phase is 114 g/min for Argon and 83 g/min for the recirculated N2. Given that the initial injection temperatures for both gases remained near ambient, it can be concluded that the cooling effect (which is approximated by multiplying the mass flow rate and the heat capacity for both gases) of the recirculated N₂ was approximately 45% larger than that of the Argon. This indicates that the recirculated N2 in the EGR stove will have a larger effect on emissions. Ultimately, literature indicates that the effect of cooling the flame in small biomass combustion applications will increase the mass emissions of particles. This may be explained by an expansion of cooler regions less than approximately 800 oC, where particle growth tends to be greater than particle oxidation.

This isolated effect of dilution of the reactive components in solid biomass combustion is not easy to discern from this test data. Additionally, this effect is not well documented in literature. Consequently, the combined effects of dilution and temperature reduction are grouped together as one emissions mechanism that us observed to cause an increase in PM_(2.5) emissions.

Firepower

In the tests using EGR at the optimized flow rates through the side injection nozzles, it was observed that the firepower unintentionally increased. This is due to the forcing of oxidizer near the biofuel surface. In order to isolate the effect of the increased firepower observed with the optimized EGR tests the expected PM emissions at the measured firepower in the EGR stove are compared with the baseline M5000 emissions using the same fuel feeding approach.

TABLE 11 Effect of Firepower Increase from Application of EGR EGR at optimized Test Description flow rates Average FP for Optimized EGR tests(kW) 2.9 Average Predicted Baseline PM_(2.5) (mg/MJ_(d)) from 310 Optimized EGR tests Average Baseline PM_(2.5) (mg/MJ_(d)) using 280 3 stick feeding approach Percent Reduction in PM_(2.5) from Predicted Baseline −12%

The results seen in Table indicate that the isolated effect of increased firepower resulting from the application of EGR led to a slight increase in PM emissions.

Understanding EGR Emissions Reductions Mechanisms General Conclusions

The experimental optimization of the EGR stove led to a reduction in PM_(2.5) mass emissions. The optimized configuration reduced emissions from a baseline value of 280 mg/MJd, to an optimized value of 150 mg/MJd. The optimized stove utilized side injection nozzles that injected recirculated exhaust gas into the oxidation zone of the flame, and forced mixing and an increase in fuel consumption rate. In order to better understand the driving forces behind net emissions reduction, potential mechanisms that can affect PM_(2.5) mass were identified and their effects were isolated experimentally.

It was determined that the mechanisms for reducing PM_(2.5) mass emissions include the chemical effect due to injecting the optimized O₂ concentration within the flame above the fuel, increased residence time of particles in the flame via recirculation and enhanced mixing. Out of the mechanisms that reduce PM_(2.5) emissions, it was shown that the chemical effect of injecting the optimized O₂ concentration is the most prominent. The isolated effect of CO₂ recirculation was determined to have no significant effect on PM_(2.5) emissions. Additionally, the combined effects of temperature reduction and dilution due to recirculated Nitrogen, and the isolated effect of the increased fuel consumption rate due to the application of EGR likely lead to increases in PM_(2.5) emissions. However, when these mechanisms effects are combined, a net reduction in PM_(2.5) mass emitted is observed.

EXAMPLE IV Comparison of EGR with Air Injection

It was found that one of the primary mechanisms for emissions reduction in the EGR stove was the chemical effect of O₂ when injected into the oxidation region of the flame. These results indicate that a stove that utilizes air injection in a similar fashion as the EGR stove side injection nozzles configuration may lead to similar or greater emissions reductions. A study was conducted to confirm this hypothesis, and to understand the relative impacts between the two fundamentally different forced draft systems.

Testing Methodology

The M5000 was used for these tests. For the forced draft air injection system, compressed air regulated by an Alicat Mass Flow Controller was routed through the side injection nozzles. The side injection nozzles were the same as those used above, with 12 holes at 4.9 millimeter diameter that inject air perpendicularly to the natural draft of the stove at the top of the combustion chamber.

In order to make a fair comparison between EGR and air injection, the minimized PM emissions for each forced draft system were determined for the side injection nozzles configuration. The minimized emissions for this configuration using EGR were previously determined above. In order to determine the minimized emissions using air injection, the same procedure, as outlined in section 4.4.3 was followed. Once the minimized emissions for each forced draft system were determined, a comparison was made.

Results/Discussion

The results of air flow rate optimization tests can be seen below in FIG. 19. The optimized flow rates using air injection were determined to be 40 and 80 SLPM for the start up and steady state phases, compared to the optimized EGR flow rates of 50 and 70 SLPM. The results of the three cold start WBTs using the optimized air flow rates, and the previously defined results of the optimized EGR flow rate tests are seen below in Table 12.

TABLE 12 Air and EGR Comparison Forced Draft Air EGR Number of Tests Completed 3 2 Mean 80% CI Mean Time to Boil (min) 21.2 19.5-23.0 28.9 Temperature Corrected Time to Boil (min) 21.2 19.3-23.1 32.1 Fuel Use to Boil (g) 307 303-311 317 FP kW 3.8 3.6-3.9 2.9 CS Thermal Efficiency (%) 35 34-35 32 PM (mg) 138 113-162 248 PM per Energy Delivered (mg/MJd) 83 71-95 150 Percent Reduction from Baseline (%) 70% — 44%

The results in Table 12 indicate that the air injection stove performed better than the EGR stove, with a total overall emissions reduction of 70% compared to 44%. As was shown above, elevation of the O₂ concentration in the oxidation region of the flame was a major factor in contributing to the emissions reduction. An air injection stove can force higher concentrations of O₂ without diluting or cooling the flame as much as an EGR stove, where the recirculated exhaust gas is partly comprised of CO₂, thus increasing the total chemical effect of O₂.

These results proved that air injection may be a practical solution for a forced draft system in a rocket elbow cookstove. Consequently, the remainder of this study focuses on further optimization of air injection methods.

In some embodiments, different nozzle configurations may perform better for EGR than for air injection, or vice versa.

EXAMPLE V Optimization of Air Injection Nozzle Diameter for Side Injection Nozzles

After the discovering that a forced draft air system will perform better or equivalently to an EGR system for the case of small rocket elbow stoves, it was decided that the study would proceed with further nozzle optimization using air injection. In section 7 the effect of changing the hole diameter of the side injection nozzles while using a fixed injection location and a fixed number of holes is explored.

Testing Methodology

The M5000 was used for these tests. For the forced draft air injection system, compressed air regulated by an Alicat Flow Controller was routed through the side injection nozzles. The side injection nozzles were located at the top of the combustion chamber, in a parallel orientation with 6 holes per nozzle.

Four different diameters were tested, including 2.3, 3.2, 4.9 and 5.7 mm. For each diameter, the flow rate optimization as described above was completed. Three cold start WBTs were then completed for each diameter at the optimized flow rates.

Results/Discussion Nozzle Diameter and Optimized Flow Rate

The major results of the flow rate optimization for each diameter can be seen in Table 13. Detailed results of flow rate sweep testing can be found at FIGS. 30-36.

TABLE 13 Nozzle Diameter Optimization Results for Side Injection Nozzles Nozzle Diameter (mm) 5.7 4.9 3.2 2.3 Optimized Start-up Flow 20 40 40 20 (SLPM) Optimized Steady-state 80 80 80 60 Flow (SLPM) Number of Cold Start Tests 3 3 3 3 80% 80% 80% 80% Mean CI Mean CI Mean CI Mean CI Time to Boil (min) 25.7 23.8-27.7 21.2 19.5-23.0 25.5 22.4-28.5 22.3 20.6-24.0 Temperature Corrected 25.7 23.8-27.7 21.2 19.3-23.1 25.5 22.6-28.4 22.3 20.8-23.8 Time to Boil (min) Fuel Use to Boil (g) 301 293-308 307 303-311 313 305-321 301 297-305 FP kW 3.1 2.8-3.3 3.8 3.6-3.9 3.2 2.9-3.5 3.6 3.4-3.9 CS Thermal Efficiency (%) 34 34-35 35 34-35 33 32-33 33 33-34 PM (mg) 144 125-162 138 113-162 131 103-160 141 97-184 Optimized PM_(2.5) 89 78-100 83 71-95 82 64-99 89 60-110 Emissions (mg/MJ) % Reduction from 68% N/A 70% N/A 70% N/A 68% N/A Baseline PM_(2.5) Emissions

The reduction in PM_(2.5) emissions from the baseline M5000 PM_(2.5) emissions data are presented for comparison. It can be seen that significant PM_(2.5) emission reductions of approximately 70% from baseline were achieved for each of the nozzle diameters tested (p=0.03, 0.02, 0.02 and 0.02 for diameters of 5.7, 4.9, 3.2 and 2.3 mm, respectively).

It can be seen that the optimized flow rates are very different between the start-up phase and steady state phase. This indicates that, as suggested above, the flow rate may be correlated with firepower.

It should also be noted that the optimized steady state flow rates tend to decrease slightly as the diameter decreases. This can be explained by the observed elevated tendency of smoke to spurt out the front of the combustion chamber at higher forced draft velocities. This effect limits the flow rates through small diameter and higher velocity nozzles.

Nozzle Diameter and PM Emissions

FIG. 20 presents the optimized PM_(2.5) emissions as a function of the nozzle diameter. The error bars represent the 80% confidence interval for each set of tests.

It can be seen that the optimized PM_(2.5) emissions are similar across the entire range of diameters tested, indicating that if the optimized flow rates are used, the various diameters will produce similar emissions reductions.

FIG. 21 shows the velocity of the injected air at the steady state flow rates for each of the diameters. Again, the error bars also represent the 80% confidence interval for each set of tests. The data points from left to right represent the 5.7, 4.9, 3.2 and 2.3 mm diameters. The diameter of the orifice in the injection nozzle may be greater than about 0.5 mm and less than about 3.5 mm. In many embodiments the diameter of the orifice in the injection nozzle may be less than about 9.0 mm, 8.0 mm, 7.0 mm, 6.0 mm, 5.0 mm, 4.5 mm, 4.0 mm, 3.5 mm, 3.0 mm, 2.9 mm, 2.8 mm, 2.7 mm, 2.6 mm, 2.5 mm, 2.4 mm, 2.3 mm, 2.2 mm, 2.1 mm, 2.0 mm, 1.9 mm, 1.8 mm, 1.7 mm, 1.6 mm, 1.5 mm, 1.4 mm, 1.3 mm, 1.2 mm, 1.0 mm, 0.9 mm, 0.8 mm, 0.7 mm, 0.6 mm, or 0.5 mm, and greater than about 0.4 mm, 0.5 mm, 0.6 mm, 0.7 mm, 0.8 mm, 0.9 mm, 1.0 mm, 1.1 mm, 1.2 mm, 1.3 mm, 1.4 mm, 1.5 mm, 1.6 mm, 1.7 mm, 1.8 mm, 1.9 mm, 2.0 mm, 2.1 mm, 2.2 mm, 2.3 mm, 2.4 mm, 2.4 mm, 2.5 mm, 2.6 mm, 2.7 mm, 2.8 mm, 2.9 mm, 3.0 mm, 3.5 mm, 4.0 mm, 5.0 mm, 6.0 mm, 7.0 mm, 8.0 mm, or 9.0 mm.

In FIG. 21, it can be seen that velocity of the injected air increases significantly as the side injection nozzle hole diameter decreases, but the optimized PM emissions remain relatively constant. This implies that for the range of diameters tested and if the optimized flow rates are used, then the velocity can vary. In many embodiments the velocity of gas exiting the injection nozzle may be from about 5 m/s to 20 m/s. In some embodiments the velocity of the gas may be greater than about 1 m/s, 2 m/s, 3 m/s, 4 m/s, 5 m/s, 6 m/s, 7 m/s, 8 m/s, 9 m/s, 10 m/s, 11 m/s, 12 m/s, 13 m/s, 14 m/s, 15 m/s, 16 m/s, 17 m/s, 18 m/s, 19 m/s, 20 m/s, or 25 m/s, and less than about 30 m/s, 25 m/s, 20 m/s, 19 m/s, 18 m/s, 17 m/s, 16 m/s, 15 m/s, 14 m/s, 13 m/s, 12 m/s, 11 m/s, 10 m/s, 9 m/s, 8 m/s, 7 m/s, 6 m/s, 5 m/s, 4 m/s, 3 m/s, or 2 m/s. Where the velocity is too low (because the injection orifice holes are too big and/or the volume of gas delivered is too low), the gas may not traverse the flame to bring additional oxidizer into the center of the flame.

Another conclusion that may be drawn from FIG. 21 is that the dispersion of the optimized emissions may increase as velocity increases. This can be explained, as previously mentioned, by the observed elevated tendency of smoke to spurt out the front of the combustion chamber at higher forced draft velocities.

Local Peak Emissions Behavior

During the flow rate tests an interesting localized peak emissions behavior was observed at a few instances. Examples of this local peak behavior are seen below at 30 SLPM in FIG. 22 and 40 SLPM in FIG. 23, where FIG. 22 displays the results of the forced draft flow rate sweep for the for the start-up phase for the 3.2 mm diameter nozzles and where FIG. 23 displays the results of the forced draft flow rate sweep for the steady-state phase for the 5.7 mm diameter nozzles.

Visual observations of the flame and fluid flow characteristics help to provide justification for these local peaks in emissions. The observed flow patterns are depicted in FIG. 24.

The flow profiles pictured on the right side are rough depictions of the flow through the combustion chamber, with side injection nozzles supplementing the flow for profiles 1 through 3 at the black arrows.

Point 0 represents an undisturbed natural draft through the combustion chamber, with no forced draft. At point 1, which would be at approximately 20 SLPM in FIG. 23 the oxidation region of the flame is supplemented therefore decreasing the PM emissions, but no mixing is occurring below the height of the side injection nozzles. At point 2, which would be at approximately 40 SLPM in FIG. 23, the forced draft is strong and concentrated enough to quench the oxidation region of the flame at the height of the nozzles, but not strong enough to induce mixing below the nozzles. The quenching of the oxidation region of the flame causes a local increase in PM emissions. At point 3, which would correspond to flow rates of 60 SLPM and greater in FIG. 23, the forced draft is strong enough to overcome the natural draft of the stove and induces mixing throughout the combustion chamber. This more evenly distributes the forced draft, allowing for higher forced draft flow rates without the consequence of quenching the flame, and ultimately causing significant reductions in PM emissions.

This effect was not observed in all of the flow rate sweeps. It may be postulated that this effect would have been observed in all of the flow rate sweeps if a finer flow rate sweep resolution had been used.

EXAMPLE VI Optimization of Air Injection Location

To further investigate the air injection method the injection location was evaluated. In this section, various injection locations are explored in the G3300.

Testing Methodology Comparison of G3300 and M5000 PM Emissions Performance

The G3300, used in these tests, is similar in design to the M5000, with a some differences. The G3300 is insulated around the combustion chamber, whereas the M5000 uses an aluminum radiation shield. The G3300 has a slightly larger combustion chamber with a wider opening than the M5000. Lastly, the G3300 ceramic base is smaller than the M5000 ceramic base. Otherwise, both stoves have similar chimney dimensions. It was found that these differences cause a difference in baseline PM_(2.5) emissions between the two stoves. However, an additional comparison was made between the performances of the stoves when a forced draft is applied. This second comparison was completed to ensure that the forced draft knowledge gained from previous work on the M5000 could be extended to work on the G3300.

A comparison between the minimized PM_(2.5) emissions and optimized flow rates for each stove was completed using the side injection nozzles with a diameter of 2.3 mm. The minimized PM_(2.5) emissions and optimized flow rates for each stove were determined using same procedure outlined above.

Injection Location

A comparison between the minimized PM_(2.5) emissions and optimized flow rates for 4 injection locations were tested, including the top of the combustion chamber and the bottom, middle and top of the chimney section. FIG. 25 presents a cross sectional view of the G3300 rocket elbow style design, with the general injection locations labeled. The distance from the top of the ceramic base (floor), in centimeters, is labeled.

In order to test the top of the combustion chamber injection location, the side injection nozzles were used. In order to test injection locations within the chimney section, a “chimney ring” nozzle was fabricated. FIG. 26 displays the chimney ring nozzle used in the lowest portion of the chimney. The chimney ring style nozzles shown in FIG. 26 inject gas horizontally and towards the vertical axis of the chimney.

For all of the nozzles in this section, 12 holes were used with 1.5 mm diameters. A diameter of 1.5 mm was selected because preliminary studies indicated that the chimney ring style nozzles with larger diameters may reduce the flame at low flow rates. This can, in some cases, cause smoke and flames to be emitted out of the front of the combustion chamber. The 1.5 mm diameter reduced this effect and allowed for higher forced draft flow rates to be used. Further discussion of the 1.5 mm diameter for the injection location study is provided below. Lastly, 12 holes were used to aid in distribution of the forced draft across the flame and to maintain consistency with previous work. In some embodiments the number of injection orifices (holes) may be greater than 12 or less than 12. In some embodiments, the injection orifices may be evenly spaced within the nozzle or may be other than evenly spaced to aid in delivering oxygen to the interior of the flame.

Results/Discussion Comparison of G3300 and M5000 PM Emissions Performance

Table 14 presents the major results of the comparison of the optimized PM_(2.5) emissions between the G3300 and M5000 when using the side injection nozzles with a 2.3 mm diameter. The mean values represent the average of 3 tests.

TABLE 14 Comparison of M5000 and G3300 Emissions Performance and Optimized Flow Rates with Similar Configuration Stove M5000 G3300 Optimized Start-up Phase Flow Rate (SLPM) 20 30 Optimized Steady-state Phase Flow Rate (SLPM) 60 60 Mean 80% CI Mean 80% CI Time to Boil (min) 22.3 20.6-24.0 22.2 21.5-22.9 Temperature Corrected Time to 22.3 20.8-23.8 22.1 21.3-22.9 Boil (min) Fuel Use to Boil (g) 301 296-305 325 306-345 FP kW 3.6 3.4-3.9 3.9 3.7-4.2 CS Thermal Efficiency (%) 33 33-34 31 30-33 PM (mg) 141  97-184 135  84-186 PM per Energy Delivered 87  60-110 82  53-110 (mg/MJd) Percent Reduction from Baseline 68% N/A 78% N/A PM (%)

It can be seen that the optimized flow rates for both stoves are similar. This indicates that the geometric differences between the two combustion chambers do not lead to significant differences or limitations for the optimized forced draft flow rates. This also implies that the optimized forced draft flow rates will be similar for both stoves and other nozzle configurations.

Additionally, it can be seen that the PM_(2.5) emissions for both stoves are similar (p=0.82). This indicates that the optimized PM_(2.5) emissions performance measured in previous work with the M5000 may be comparable to optimized PM_(2.5) emissions performance with the G3300, and that the differences between the two stoves performance is not so apparent when the optimized flow rate is applied.

Injection Location

Table 15 presents the major results of the injection location study. More detailed results of flow rate sweep testing can be found in FIGS. 30-36.

TABLE 15 Injection Location Optimization Results Injection Location (cm) Top of Chamber Bottom of Chimney Middle of Chimney Top of Chimney (8.7) (12.1) (17.8) (22.9) Start-up Flow (SLPM) 20 20 20 10 Steady-state Flow (SLPM) 40 40 40 10 Number of Tests 3 3 3 3 80% 80% 80% 80% Mean CI Mean CI Mean CI Mean CI Time to Boil (min) 21.1 20.5-21.7 15.0 13.5-16.6 19.3 16.5-22.1 21.8 20.5-23.1 Temperature Corrected 21.2 20.3-22.0 15.0 13.4-16.7 19.3 16.6-22.1 21.8 20.5-23.0 Time to Boil (min) Fuel Use to Boil (g) 304 287-322 320 293-346 331 310-351 276 271-281 FP kW 3.9 3.7-4.1 5.9 5.4-6.3 4.6 4.1-5.2 3.4 3.2-3.6 CS Thermal Efficiency (%) 33 31-34 31 28-34 30 28-33 36 35-37 PM (mg) 115  62-168 101  68-133 147  79 -214 449 399-498 PM_(2.5) Emissions (mg/MJ) 72  40-100 61 40-82 92  48-130 280 250-310 % Reduction from 81% N/A 84% N/A 76% N/A 26% N/A Baseline PM_(2.5) Emissions

It can be seen that the optimized flow rates are similar across the bottom three injection locations. However, at the top injection location, the optimized flow rates drop down to 10 SLPM. FIG. 27 is composed of images taken during the flow rate sweeps for the top injection location.

The disclosed devices, methods, and systems may aid in reducing pollution from a biomass stove. As demonstrated by the examples above and below, the present disclosure may aid in reducing particulate mass emissions from a biomass stove, for example PM_(2.5) emissions, relative to the same stove where gas is not being actively injected into the flame (i.e. the fan or blower is off, or no injection system is installed). In some cases, the present disclosure provides for reduction of PM_(2.5) emissions from between about 20% to about 95%. In many embodiments, the reduction of PM_(2.5) emissions is between about 25% and 85%. In some embodiments, the reduction in PM_(2.5) emissions is greater than about 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, or 90%, and less than about 100%, 95%, 90%, 85%, 80%, 75%, 70%, 65%, 60%, 55%, 50%, 45%, 40%, 35%, 30%, and 25%.

At a flow rate of 0 SLPM, the flame out of the top of the chimney is tall and wispy. At a flow rate of 10 SLPM the flame is shorter and more condensed. When a flow rate of 20 SLPM is used, the top of the flame is entirely quenched at the injection location. This causes a significant reduction in the draft through the stove, and leads to significant smoke and flame out of the front of the stove. Ultimately, this limits the optimized flow rate for the top of the chimney injection location to 10 SLPM, also limiting the PM_(2.5) emissions reduction potential for this configuration. Although this phenomenon was less significant for embodiments wherein the ring nozzle is positioned lower in the combustion chamber, because the flame is stronger at lower positions, it is less susceptible to the effect seen in FIG. 27. Rather, at lower levels the injection gas entered the oxidation region of the flame (rather than quenching it) to deliver oxygen, and promote additional oxidation and PM reduction.

Comparing the optimized PM emissions from the bottom three injection locations leads to the conclusion that bottom of the chimney is the optimized injection location. However, injection at the top of the combustion chamber also led to good performance. It may be hypothesized that these locations led to the largest reduction because they inject where the flame is the strongest and supplement the oxidation of particles without causing cooling or quenching of the flame.

Another important observation is the effect on time to boil and firepower for each injection location. It appears that the bottom of the chimney injection location leads to significant increase in firepower and a large decrease in time to boil, both features that may be quite valuable to the consumer.

Ultimately, injecting the forced draft at the bottom of the chimney may be desirable. This placement may aid in emissions reductions from baseline (p=0.001) and may provide for a relatively inconspicuous design when compared to the side injection nozzles, which intrude into the combustion chamber. In many embodiments, placement of the injection nozzle promotes gas injection above the level of solid biofuel in the combustion chamber. In most embodiments, the gas is injected between 0.5 and 30.0 cm above the solid fuel (e.g. the wood seen in FIG. 26). In most embodiments, the gas is injected into the flame more than about 0.5 cm, 1.0 cm, 1.5 cm, 2.0 cm, 2.5 cm, 3.0 cm, 3.5 cm, 4.0 cm, 4.5 cm, 5.0 cm, 5.5 cm, 6.0 cm, 6.5 cm, 7.0 cm, 7.5 cm, 8.0 cm, 8.5 cm, 9.0 cm, 9.5 cm, 10 cm, 11 cm, 12 cm, 13 cm, 14 cm, 15 cm, 16 cm, 17 cm, 18 cm, 19 cm, 20 cm, or 25 cm, and less than about 30 cm, 25 cm, 20 cm, 19 cm, 18 cm, 17 cm, 16 cm, 15 cm, 14 cm, 13 cm, 12 cm, 11 cm, 10 cm, 9.5 cm, 9.0 cm, 8.5 cm, 8.0 cm, 7.5 cm, 7.0 cm, 6.5 cm, 6.0 cm, 5.5 cm, 5.0 cm, 4.5 cm, 4.0 cm, 3.5 cm, 3.0 cm, 2.5 cm, 2.0 cm, 1.5 cm, or 1.0 cm.

EXAMPLE VII Optimization of Air Injection Nozzle Diameter for Chimney Ring Nozzle

Above, it was found that the diameter of the nozzle is not a strong determinant of the minimized PM emissions when the injection location is at the top of the combustion chamber, with the side injection nozzles. However, the minimized PM emissions may not be a weak function of diameter for other injection locations and nozzle configurations. Thus, in this section the effect diameter has on emissions for the chimney ring style nozzle located at the bottom of the chimney is explored.

Testing Methodology

The effect of nozzle diameter on emissions was tested for the chimney ring nozzle located at the bottom of the chimney in the G3300. Hole diameters of 1.5 and 3.0 millimeters were tested. For each diameter, the flow rate optimization as described above was completed. Three cold start WBTs were then completed for each diameter at the optimized flow rates. The resulting optimized emissions and flow rates were then compared.

Results/Discussion

The results of the diameter optimization for the chimney ring style nozzle are displayed below in Table 16.

TABLE 16 Diameter Optimization Results for Bottom of Chimney Injection Location Nozzle Diameter (mm) 1.5 3.0 Optimized Start-up Phase Flow Rate (SLPM) 20 20 Optimized Steady-state Phase Flow Rate (SLPM) 40 20 Mean 80% CI Mean 80% CI Time to Boil (min) 15.0 13.5-16.6 23.3 22.7-23.9 Temperature Corrected 15.0 13.4-16.7 23.5 22.9-24.1 Time to Boil (min) Fuel Use to Boil (g) 320 293-346 283 280-285 FP kW 5.9 5.4-6.3 3.2 3.2-3.3 CS Thermal Efficiency 31 28-34 35 35-36 (%) PM (mg) 101  68-133 167 140-195 PM per Energy Delivered 61 40-82 100  87-120 (mg/MJd) Percent Reduction from 84% N/A 72% N/A PM Baseline

It can be seen that the 1.5 mm diameter nozzles had a larger optimized steady-state flow rate. This is because the 3.0 mm diameter nozzle caused quenching of the flame at flow rates greater than 20 SLPM, leading to the emissions of smoke and flames out of the front of the combustion chamber. It can also be seen that the optimized PM_(2.5) emissions were significantly lower for the 1.5 mm diameter than for the 3.0 mm diameter (p=0.04). The quenching of the flame limited the forced draft flow rate which consequently limited the PM_(2.5) emissions reduction.

This data indicates that for the chimney ring style nozzle, the may be diameter is less than about 3 mm.

EXAMPLE VIII Optimization of Air Injection Angle for Chimney Ring Nozzle

Above, the injection location was investigated in addition to diameter. Here, the effect of altering the angle of injection for the injection location and diameter is explored.

Testing Methodology

In order to understand the effect of injection angle the minimized emissions from two configurations were compared. The injection location used for these tests was at the bottom of the chimney, the optimal injection location found in section 8. The tested injection angles were either horizontal or 30° above horizontal. FIG. 28 displays the location of the two nozzles used in this study. Additionally, both nozzles used in this study had 12 holes with 1.5 mm diameters. FIG. 28 shows injection angles tested at bottom of chimney. FIG. 29 shows the chimney ring nozzle that injects at an angle of 30° from horizontal.

Results/Discussion

Table 17 displays the results of the injection angle study. More detailed results of flow rate sweep testing can be found in FIGS. 30-36.

TABLE 17 Injection Angle Test Results Injection Angle - Location Horizontal - 30° upwards from Bottom of Horizontal - Bottom Chimney of Chimney Optimized Start-up 20 10 Phase Flow Rate (SLPM) Optimized Steady-state 40 10 Phase Flow Rate (SLPM) Mean 80% CI Mean 80% CI Time to Boil (min) 15.0 13.5-16.6 23.7 23.0-24.5 Temperature Corrected 15.0 13.4-16.7 23.9 23.2-24.6 Time to Boil (min) Fuel Use to Boil (g) 320 293-346 276 263-290 FP kW 5.9 5.4-6.3 3.1 3.1-3.2 CS Thermal Efficiency 31 28-34 36 35-37 (%) PM (mg) 101  68-133 199 186-211 PM per Energy Delivered 61 40-82 130 120-130 (mg/MJd) Percent Reduction from 84% N/A 67% N/A PM Baseline

It can be seen that the optimized flow rates for the 30° injection angle is limited to 10 SLPM. Additionally, the emissions reduction achieved with the horizontal injection angle is greater than the emissions reduction achieved with a 30° injection angle (p=0.02).

A lesser emissions reduction is achieved with the angled injection because the angle of the forced draft promotes a high total draft through stove. Increasing the total draft through the stove can cool the flame significantly, especially if there is not a high level of mixing. Cooling of the flame can increase the volume of regions of particle growth and decrease the regions of particle oxidation.

While a horizontal injection angle is preferred, the angle of injection may vary between about −50° (from horizontal, 0°; i.e. downward from the nozzle toward the fuel) to about +50° (from horizontal, 0°, i.e upward toward the top of the upper combustion chamber), preferably from about −30° to about +30°. In many embodiments the injection angle may be greater than about −55°, −45°, −40°, −45°, −30°, −25°, −20°, −15°, −10°, −9°, −8°, −7°, −6°, −5°, −4°, −3°, −2°, −1°, 0° (horizontal), 1°, 2°, 3°, 4°, 5°, 6°, 7°, 8°, 9°, 10°, 11°, 12°, 13°, 14°, 15°, 16°, 17°, 18°, 19°, 20°, 25°, 30°, 35°, 40°, 45°, or 50°, and less than about 55°, 50°, 45°, 40°, 35°, 30°, 25°, 20°, 19°, 18°, 17°, 16°, 15°, 14°, 13°, 12°, 11°, 10°, 9°, 8°, 7°, 6°, 5°, 4°, 3°, 2°, 1°, 0°, −1°, −2°, −3°, −4°, −5°, −6°, −7°, −8°, −9°, −10°, −11°, −12°, −13°, −14°, −15°, −16°, −17°, −18°, 19°, −20°, −25°, −30°, −35°, −40°, −45°, −50°, or −55°. 

We claim:
 1. A device for reducing emissions from a biomass stove, the device comprising: a fluid inlet orifice; an inlet conduit having an outside surface and an interior surface, the interior surface defining an inlet chamber, the inlet chamber in fluid communication with the exterior surface via the inlet orifice, the interior chamber for channeling a fluid; a fan positioned within the inlet chamber and distal the inlet orifice, the fan for drawing a fluid through the inlet orifice and into the chamber, and into; an outlet conduit the outlet conduit having an interior surface defining an outlet chamber, the outlet chamber in fluid communication with the inlet chamber; one or more nozzles having an interior in fluid communication with the outlet chamber, the nozzle for directing the fluid into a combustion chamber of a biomass stove; and a plurality of outlet orifices defined on the surface of the nozzle, the outlet orifices designed to allow the fluid to exit the interior of the nozzle.
 2. The device of claim 1, wherein the nozzle is positioned at or near the top of a lower combustion chamber.
 3. The device of claim 1, wherein the outlet orifices have an average diameter of between 0.5 and 3.5 mm.
 4. The device of claim 3, wherein the outlet orifices define a circle, a square, a triangle, or an oval, the average diameter being measured through the center of the circle, square, triangle, or oval.
 5. The device of claim 3, wherein the volume of gas escaping the one or more nozzles is greater than about 10 standard liters per minute and less than about 100 standard liters per minute.
 6. The device of claim 5, wherein the velocity of gas escaping from the one or more nozzles is greater than about 5 meters per second and less than about 20 meters per second.
 7. The device of claim 1, wherein the nozzle is selected from linear or circular.
 8. The device of claim 7, wherein the nozzle is a ring positioned above the lower combustion chamber and within the lower half of the upper combustion chamber, and designed to allow combustion gasses to pass directly through an injection region.
 9. The device of claim 1, wherein the fluid is a gas comprising greater than about 15% O₂, oxygen.
 10. A method of reducing emissions from a biomass stove, the method comprising: placing a gas into an interior chamber of nozzle, the nozzle positioned at or near a flame; increasing the pressure of the gas within the nozzle; expelling a volume of the gas from the nozzle through a plurality of outlet orifices defined by the outer surface of the nozzle; and directing the injected gas into a flame within a combustion chamber of the biomass stove, wherein the gas decreases the amount of at least one pollutant exiting the biomass stove.
 11. The method of claim 10, wherein the volume of gas is between about 10 standard liters per minute and 100 standard liters per minute.
 12. The method of claim 10, wherein the nozzle defines a ring, and the outlet orifices are positioned in the interior surface of the ring to aid in injecting a gas into the center of the ring.
 13. The method of claim 10, wherein the outlet orifices define a diameter of between 0.5 and 6.0 mm.
 14. The method of claim 10, wherein the outlet orifices are positioned equidistant from a floor of the combustion chamber.
 15. The method of claim 10, wherein the gas is forced into the nozzle by an electrically powered fan or blower.
 16. The method of claim 10, wherein the gas is expelled through one or more orifices at a velocity of between 5 and 25 meters per second.
 17. The method of claim 10, wherein the emission reduced is particulate matter less than about 2.5 micrometer.
 18. The method of claim 17, wherein the amount of particulate matter, less than about 2.5 micrometer, emitted from the stove during combustion is between about 25% and 90% less than the amount emitted when the pressure of the gas in the nozzle is not increased.
 19. The method of claim 1, wherein the gas is injected into the flame at an angle of between about −10 degrees to about +30 degrees.
 20. A method of reducing particulate emissions from a biomass stove, the method comprising: drawing a gas into a chamber, the gas comprising greater than about 15% O₂; channeling the gas from the chamber into a nozzle having an interior surface and an exterior surface, the nozzle defining a circular tube having a plurality of outlet orifices on the inner surface of the circle, wherein the outlet orifices allow a gas to travel from the interior of the tube and toward the center of the circle; increasing the pressure of the gas within the interior of the nozzle; expelling a volume of the pressurized gas from the nozzle at a velocity of between about 5 meters per second and 20 meters per second; and directing the injected gas into a flame within a combustion chamber of the biomass stove, wherein the gas decreases the amount of at least one pollutant exiting the biomass stove by greater than about 25% relative the stove lacking a nozzle or lacking a pressurized gas within the nozzle. 